Television tuning circuits have been known, developed and improved since the advent of the television. Such tuning circuits convert received high frequency radio frequencies to an intermediate frequency (“IF”). Historically, such tuning circuits were embodied in tuners that formed integral parts of single purpose bulky television receivers, typically including a cathode ray tube. These tuners were usually manufactured using conventional discrete passive and active components. Accordingly, tuners had traditionally been quite bulky and expensive to produce.
More recently, television receivers have formed part of smaller electronic devices such as handheld televisions, video cassette recorders (“VCR”s), and personal computing devices. As a result, much has been done to miniaturize television tuning circuits and form such circuits using integrated circuits. For example, U.S. Pat. No. 5,737,035 to Rotzoll discloses a television tuning circuit integrally formed on an integrated circuit substrate.
Existing integrated tuning circuit designs, however, rely on multiple loop frequency synthesizers in order to allow for fine tuning, as for example detailed in U.S. Pat. No. 5,847,612 to Birleson. Multiple loop frequency synthesizers, however, typically only allow for limited fine tuning. Moreover, such multiple loop frequency synthesizers rely on multiple phase locked loops (“PLL”s) and independent voltage controlled oscillators (“VCO”s) co-located on a single integrated circuit substrate.
As such, much care must be taken to prevent the co-located VCOs from interfering with the received signal and with each other. Clearly, the more such VCOs that are used, the more difficult it is to prevent the interference of spurious signals. Current integration techniques, however, typically do not entirely prevent these multiple PLLs and VCOs from interfering with each other.
Accordingly, tuning circuits that reduce the dependence on multiple loop frequency synthesizers and allow improved fine tuning are desirable.
As well, recent television designs have also made concurrent viewing of several channels possible. Such designs typically overlay one picture in another, or present a picture-in-picture (“PIP”). This is made possible through the use of two television tuners providing IF signals representative of each channel to a picture-in-picture mixing circuit. Conventionally, two single tuning circuits are used, one for each IF signal.
The use of two single tuning circuits, of course, requires double the number of components as that required for a single tuning circuit; twice the power consumption; twice the space; and additional assembly time.
Accordingly, integration of multiple tuning circuits would also be desirable.